1. Field of the Invention
The present invention relates to a method of controlling the shifting of a hydraulically-actuated type multi-step transmission for use on a large-sized construction vehicle and the like.
2. Description of the Prior Art
A hydraulically-actuated multi-step transmissions which are equipped with a plurality of hydraulic clutches and gear trains and wherein a shift can be made to a desired speed gear by engaging a specific hydraulic clutch or clutches, are known.
These type of hydraulically-actuated multi-step transmission are used in combination with a torque converter for increasing the maximum tractive power during vehicle starts.
On the other hand, since a large-sized construction vehicle, such as a large-sized dump truck, requires a large amount of torque for high-speed traveling, a torque converter having a direct-coupled or lock-up clutch is provided. In these type of arrangements the pump and turbine of the torque converter can be selectively connected by the direct-coupled clutch for efficient transmission of large amounts of torque. During shifting, however, the direct-coupled clutch is disengaged for reducing the load of the hydraulic clutches of the transmission. After the shift is concluded, the direct-coupled clutch is re-engaged.
That is to say, if a shift is made with the direct-coupled clutch left engaged, a great load is exerted on at least one of the hydraulic clutches of the transmission, resulting in the generation of severe shift shock. During shifting, therefore, the direct-coupled clutch is disengaged to lessen the load applied to the hydraulic clutches, thereby decreasing the shift shock.
Increasing the capacity of the hydraulic clutches of the transmission can decrease, to some degree, the effect of the load acting thereon during operation. However, the transmission incorporating hydraulic clutches of increased capacity become large in size and costly.
According to this method of controlling speed changes, however, there occur both a shift shock during shifting due to the engagement of the hydraulic clutches and an engagement shock at the time of engagement of the direct-coupled clutch. This increases the amount of shock at the time of shifting, and causes the driver discomfort.
A markedly great engagement shock is produced when the direct-coupled clutch is engaged and the transmission is conditioned to produce low gear.